Poxviruses
Poxviruses have been previously exploited as recombinant vectors for the heterologous expression of foreign proteins. In particular, recombinant vaccinia virus has been studied as a tool for transient expression of genes in mammalian cells and an experimental recombinant vaccine vector (reviewed by Moss, 1991, Proc. Natl. Acad. Sci., USA 93, 11341-8; and Moss, 1996, Proc. Natl. Acad. Sci., USA 93, 11341-8).
In common with the other poxviruses, vaccinia virus resides within the cell cytoplasm where it expresses the proteins needed for viral replication. Recombinant vaccinia can, therefore, deliver foreign antigens to the cytoplasm of mammalian cells, thereby allowing them direct access to antigen processing pathways which leads to presentation of antigen derived peptides in association with MHC Class I and Class II molecules on the cell surface (Moss, 1991, Proc. Natl. Acad. Sci., USA 93, 11341-8). This property makes vaccinia useful as recombinant vaccines, particularly for stimulating CD8+ and CD4+ T-cell immune responses.
Concern about the capacity of vaccinia virus to replicate in mammalian cells has limited its clinical use and led to the search for safer alternatives. These include attenuated vaccinia viruses, such as modified vaccinia Ankara (MVA) (Meyer et al, 1991, J. Gen. Virol. 72, 1031-8; Sutter and Moss, 1992, Proc. Natl. Acad. Sci., USA 89, 10847-51; Sutter et al, 1994, Vaccine 12, 1032-40), which undergoes limited replication in human cells (Blanchard et al, 1998, J. Gen. Virol. 79, 1159-67), and the avipox viruses, such as fowlpox, which do not proliferate in mammalian cells (Somogyi et al, 1993, Virology 197, 439-44).
Wild-type fowlpox viruses, which cause proliferative skin lesions that are rarely lethal in birds, are of commercial concern in the poultry industry. Live attenuated vaccines against fowlpox virus have been produced by multiple passage of the virus in avian cells. Such attenuated fowlpox viruses expressing antigens from poultry pathogens have been extensively exploited as recombinant vaccines for avian use (reviewed by Boyle and Heine, 1993, Immunol Cell Biol 71, 391-7; Paoletti, 1996, Proc. Natl. Acad. Sci., USA 93, 11349-53). In fact, two recombinant fowlpox viruses expressing antigens from Newcastle's disease virus are commercially available for veterinary use in the USA (Paoletti, 1996, Proc. Natl. Acad. Sci., USA 93, 11349-53).
The observation that avipox viruses can express antigens in mammalian cells and can induce a protective immune response against mammalian pathogens (Taylor and Paoletti, 1998, Vaccine 6, 466-8; Taylor et al, 1998a, Vaccine 6, 504-8; Taylor et al, 1988b, Vaccine 6, 497-503), led to the development of recombinant fowlpox viruses as vaccines for use in mammals. Most significantly, recombinant fowlpox expressing antigens from HIV have shown promise as vaccines in non-human primates (Dale et al, 2000, J Med Primatol 29, 240-7; Kent et al, 1988, J Virol 72, 10180-8; Kent et al, 2000, Vaccine 18, 2250-6). In addition, recombinant fowlpox vaccines encoding tumour-associated antigens have been evaluated in animals (Grosenbach et al, 2001, Cancer Res 61, 4497-505; Irvine et al, 1997, J Natl Cancer Inst 89, 1595-601; Wang et al, 1995, J Immunol 154, 4685-92) and are presently undergoing human clinical trials.
The majority of attenuated fowlpox vaccine strains are not fully defined in terms of their genome organisation and exact sequence. In fact, the genomes of some have recently been found to carry an infectious copy of the provirus of avian reticuloendotheliosis virus (REV) (Hertig et al, 1997, Virology 235, 367-76) which may limit their use as recombinant vectors.
There is an upper limit on genome size for vectors derived from pox viruses. For vaccinia, it is thought that the maximum size of heterologous sequence that can be effectively packaged and delivered is 10% of the size of the genome.
Thus there is a need for an improved vector system, which lacks the capacity to replicate in mammalian cells, but which is better characterised, is better at eliciting T-cell immune responses, has an improved capacity to accommodate and deliver heterologous DNA and/or has improved safety over known attenuated fowlpox vaccine strains.
Vaccination Strategies
There are numerous methods known in the art to stimulate an immune response in a subject in order to prevent and/or treat a disease. Examples of antigenic preparations used as vaccines are shown in the following table (Table 1).
TABLE 1Type of antigenVaccine examplesLivingNaturalVaccinia (for small pox)organismsVole bacillus (for TB)attenuatedPolio (Sabin; oral polio vaccine)Measles, mumps, rubella, yellow fever17dVaricella-zoster (human herpes virus 3)BCG (for TB)Intact butvirusesPolio (Salk), rabies, influenza, hepatitisnon-livingA,organismstyphusbacteriaPertussis, typhoid, cholera, plagueSubcellularCapsularPneumococcus, meningococcus,fragmentspolysaccharidesHaemophilus influenzaeSurface antigenHepatitis BToxoidsTetanus, diphtheriaRecombinantGene clonedHepatitis B (yeast derived)DNA-Basedand expressedGene expressedin vectorsNaked DNAAnti-idiotype
There are also other types of non-antigen based immunisation, which include passive immunisation (the direct administration of antibodies) and non-specific immunisation (such ad by the administration or cytokines or cytokine inhibitors).
A problem with many of these approaches is that the immune response wanes over time, such that it is no longer effective, for example in controlling or eradicating an infection.
It is important that a vaccine induces the right sort of immune response for the disease. Many known vaccines are useful for generating antibodies, but do not induce significant cell-mediated immune responses. A number of diseases are particularly susceptible to prevention and/or treatment by a T cell immune response. For example, cytolytic CD8+ T cells may protect against or help to clear viral infections. Also, in the case of diseases such as tuberculosis, malaria and H. pylori infection there is evidence for a protective role for CD4+ T cells which can secrete IFNγ.
Some known viral vaccination strategies are associates with a number of complications and side effects. For example smallpox vaccination can cause generalised vaccinia, eczema vaccinatum, progressive vaccinia, and neurological and cardiac complications (Feery (1977) Med J. Aust 6 180-183; Goldstein et al (1975) Pediatrics, 55, 342-7).
There is thus need for improved vaccination strategies, particularly those capable of stimulating or boosting the T-cell arm of the immune system which cause a minimum of adverse reactions.